SPINAL CORD 677
The neurone Z may well, therefore, be the field of coalition, and the organ where the summational and inductive processes occur. And the morphology of the neurone as a whole is seen to be just such as we should expect, arguing from the principle of the common path.
With the phenomenon of " interference " the question is more difficult. There it is not clear that the field of antagonism is within the neurone Z itself. The field may be synaptic. We have the demonstration by Verworn that the interference produced by A at Z for impulses from B is not accompanied by any obvious change in excitability of the axone of Z. Z, if itself the seat of inhibition, might have been expected to exhibit that inhibition throughout its extent. This, as tested by its axone, it does not. There exist, it is true, older experiments by Uspensky, Belmondo and Oddi, &c.,' according to which the threshold of direct excitability of the motor root is lowered by stimulation of the afferent root. This points to an extension of the facilitation effect through the whole motor neurone, conversely to Verworn's demonstration for central inhibition. Verworn's experiment and its result is very clear. It leads us to search for some other mechanism common to A and B to which might be attributable their mutual influence on each other's reactions. But if we admit the conception, argued above,. that at the nexus between A and Z, i.e. at synapse A Z, and similarly between B and Z, i.e. at synapse B Z, there exists a surface of separation, a membrane in the physical sense, a further consequence seems inferable. Suppose a number of different neurones A. B, C, &c., each conducting through its own synapse upon a neurone Z. The synapses A Z, B Z, C Z, &c. are all surfaces or membranes into which Z enters as a factor common to them all. A change of state induced in neurone Z might be expected to affect the surface condition or membrane at all of the synapses, since the condition of Z is a factor common to all those membranes. Therefore a change of state (excitatory or inhibitory) induced in Z by any of the neurones A, B, C, &c., playing upon it would enter as a condition into the nervous transmission at the other synapses from the other collateral neurones. In harmony with this is the spread of refractory state in the neurones as mentioned above. A change in neurone Z induced by neurone A playing upon it, in that case seems to effect its point of nexus with the other neurones B, C, &c., also. It is conceivable that the phenomena of interference may be based in part at least on such a condition. The neurone threshold of Z for stimulation through B will be to some extent a function of events at synapses A Z.
Factors Determining the Sequence.—The formation of a common path from tributary converging afferent arcs is important because it gives a co-ordinating mechanism. There the dominant action of one afferent arc, or set of allied arcs in condominium, is subject to supercession by another afferent arc, or set of allied arcs, and the supercession normally occurs without intercurrent confusion. Whatever be the nature of the physiological process occurring between the competing reflexes for dominance over the common path, the issue of their competition namely, the determination of which one of the competing arcs shall for the time being reign over the common path, is largely conditioned by four factors. These are spinal induction; relative fatigue, relative intensity of stimulus, and the functional species of the reflex.
i. The first of these occurs in two forms, one of which has already been considered, namely, immediate induction. It is a form of " bahnung." The stimulus which excites a reflex tends by central spread to facilitate and lower the threshold for reflexes allied to that which it particularly excites. A constellation of reflexes thus tends to be formed which reinforce each other, so that the reflex is supported by allied accessory reflexes, or if the prepotent stimulus shifts, allied arcs are by the induction particularly prepared to be responsive to it or to a similar stimulus.
Immediate induction only occurs between allied reflexes. Its tendency in the competition between afferent arcs is to fortify the reflex just established, or, if transition occur, to favourtransition to an allied reflex. Immediate induction seems to obtain with highest intensity at the outset of a reflex, or at least near its commencement. It does not appear to persist long.
The other form of spinal induction is what may be termed successive induction. It is in several ways the reverse of the preceding.
In peripheral inhibition, exemplified by the vagtis action on the heart, the inhibitory effect is followed by a rebound after-effect opposite to the inhibitory (Gaskell). The same thing is obvious in various instances of the reciprocal inhibition of the spinal centres. Thus, if the crossed-extension reflex of the limb of the " spinal " dog be elicited at regular intervals, say once a minute, by a carefully adjusted electrical stimulus of defined duration and intensity, the resulting reflex movements are repeated each time with much constancy of character, amplitude and duration. If in one of the intervals a strong prolonged (e.g. 30") flexion-reflex is induced from the limb yielding the extensor-reflex movement, the latter reflex is found intensified after the intercurrent flexion-reflex. The intercalated flexion-reflex lowers the threshold for the aftercoming extension-reflexes, and especially increases their after-discharge. This effect may endure, progressively diminishing, through four or five minutes, as tested by the extensor reflexes at successive intervals. Now, as we have seen, during the flexion-reflex the extensor arcs were inhibited: after the flexion-reflex these arcs are in this case evidently in a phase of exalted excitability. The phenomenon presents obvious analogy to visual contrast. If visual brightness be regarded as analogous to the activity of spinal discharge, and visual darkness analogous to absence of spinal discharge, this reciprocal spinal action in the example mentioned has a close counterpart in the well-known experiment where a white disk used as a prolonged stimulus leaves as visual after-effect a grey image surrounded by a bright ring (Hering's "Lichthof "). This bright ring has for its spinal equivalent. the discharge from the adjacent reciprocally correlated spinal centre. The exaltation after-effect may ensue with such intensity that simple discontinuance of the stimulus maintaining one reflex is immediately followed by " spontaneous " appearance of the antagonistic reflex. Thus the flexion-reflex, if intense and prolonged, may, directly its own exciting stimulus is discontinued, be succeeded by a " spontaneous " reflex of extension, and this even when the animal is lying on its side and the limb horizontal—a pose that does not favour the tonus of the extensor muscles. Such a " spontaneous " reflex is the spinal counterpart of the visual " Lichthof." To this spinal induction, as it may be termed, seems attributable a phenomenon commonly met in a flexion-reflex of high intensity when maintained by very pro-longed excitation. The reflex flexion is then frequently broken at irregular intervals by sudden extension movements. It would seem, therefore, that some process in the flexion-reflex leads to exaltation of the activity of the arcs of the opposed extension-reflex. An electrical stimulation of the proximal end of the severed nerve of the extensor muscles of the knee (cat), though it does not, in the present writer's experience, directly excite contraction of the extensors of the knee is on cessation often immediately followed by contraction of them.
As examples of the rebound exaltation following on inhibition the following may also serve. The so-called " mark-time " reflex of the " spinal " dog is an alternating stepping movement of the hind limbs which occurs on holding the animal up so that its limbs hang pendent. It can be inhibited by stimulating the' skin of the tail. On cessation of that stimulus the stepping movement sets in more vigorously and at quicker. rate than before. The increase is chiefly in the amplitude of the movement, but the writer has also seen the rhythm quickened even by 30% of the frequence.
This after-increase might be explicable in either of two ways. It might be due to the mere repose of the reflex centre, the repose so recruiting the centre as to strengthen its subsequent action. But a similar period of repose obtained by simply supporting one limb—which causes cessation of the reflex in both limbs, the
Spinal Induction.
stimulus being stretch of the hip-flexors under gravity--is not followed by after-increase of the reflex.
Or the after-increase might result from the inhibition being followed by a rebound to superactivity. This latter seems to be the case. The after.-increase occurs even when both hind limbs are passively lifted from below during the whole duration of the inhibitory stimulus applied to the tail. It is the depression of inhibition, and not the mere freedom from an exciting stimulus, that induces a later superactivity. And the reflex inhibition of the knee-extensor by stimulation of the central end of. its own nerve is especially followed by marked rebound to superactivity of the extensor itself.
Again, the knee jerk, after being inhibited by stimulation of the hamstring nerve, returns, and is then more brisk than before the inhibition.
By virtue of this spinal contrast, therefore, the extension-reflex predisposes to and may actually induce a flexion-reflex, and conversely the flexion-reflex predisposes to and may actually induce an extension-reflex. This process is qualified to play a part in linking reflexes together in a co-ordinate sequence of successive combination. If a reflex arc A during its own activity temporarily checks that of an opposed reflex arc B, but as a subsequent result induces in arc B a phase of greater excitability and capacity for discharge, it predisposes the spinal organ for a second reflex opposite in character to its own in immediate succession to itself. The writer has elsewhere pointed out the peculiar prominence of " alternating reflexes " in pro-longed spinal reactions. It is significant that they are usually cut short with ease by mere passive mechanical interruption of the alternating movement in progress. It seems that each step of the reflex movement tends to excite by spinal induction the step next succeeding itself.
Much of the reflex action of the limb that can be studied in the " spinal " dog bears the character of adaptation to locomotion. This has been shown recently with particular clearness by the observations of Phillipson. In describing the extensor thrust of the limb the writer drew attention at the time to its significance for locomotion. Spinal induction obviously tends to connect to this extensor-thrust flexion of the limb as an after-effect. In the stepping of the limb. the flexion that raises the foot and carries it clear,of the ground prepares the antagonistic arcs of extension, and, so to say, sensitizes them to respond later in their turn by the supporting and propulsive extension of the limb necessary for progression. In such reflex sequences an antecedent reflex would thus not only be the means of bringing about an ensuing stimulus for the next reflex, but would pre-dispose the arc of the next reflex to react to the stimulus when it arrives, or even induce the reflex without external stimulus. The reflex " stepping " of the " spinal " dog does go on even without an external skin stimulus: it will continue when the dog is held in the air. The cat walks well when anaesthetic in the soles of all four feet.
Each reflex movement must of itself generate stimuli to afferent apparatus in many parts and organs—muscles, joints, tendons &c. This probably reinforces the reflex in progress. The reflex obtainable by stimulation of the afferent nerve of the flexor muscles of the knee excites those muscles to contraction and inhibits their antagonistics: the reflex obtainable from the afferent nerve of the extensor muscles of the knee excites the flexors and inhibits their antagonistics.
Where a reflex by spinal induction tends to eventually bring about the opposed reflex, the process of spinal induction is therefore probably reinforced by the operation of any reflex generated in the movement. This would help to explain how it is that a reflex reaction, when once excited in a " spinal " animal, ceases on cessation of the stimulus as quickly as it generally does. Such a reaction must generate in its progress a number of further stimuli and throw up a shower of centripetal impulses from the moving muscles and joints into the spinal cord. Squeezing of muscles and stimulation of their afferent nerves and those of joints, &c., elicit reflexes. The primary reflex movement might be expected, therefore, of itself to initiate further reflexmovement, and that secondarily to initiate further still, and so on. Yet on cessation of the external stimulus to the foot in the flexion-reflex the whole reflex comes, usually at once to an end. The scratch-reflex, even when violently provoked, ceases usually within two seconds of the discontinuance of the external stimulus that provoked it.
We have as yet no satisfactory explanation of this. But we remember that such reflexes are intercurrent reactions breaking in on a condition of neural equilibrium itself reflex. The successive induction will tend to induce a compensatory reflex, which brings the moving parts back again to the original position of equilibrium.
2. Another condition influencing the issue of competition between reflexes of different source for possession of one and the same final common path is fatigue. A spinal reflex Fatigue. under continuous excitation or frequent repetition
becomes weaker, and may cease altogether. This decline is progressive, and takes place earlier in some kinds of reflexes than it does in others. In the " spinal " dog the scratch-reflex under ordinary circumstances tires much more rapidly than does the flexion-reflex.
A reflex as it tires shows other changes besides decline in amplitude of contraction. Thus in the flexion-reflex, the original steadiness of the contraction decreases; it becomes tremulous, and the tremor becomes progressively more marked and more irregular. The rhythm of the tremor in the writer's observations has often been about to per second. Then phases of greater tremor tend to alternate with phases of improved con-traction as indicated by some regain of original extent of flexion of limb and diminished tremor. Apart from these partial evanescent recoveries the decline is progressive. Later, the stimulation being maintained all the time, brief periods of something like complete intermission of the reflex appear, and even of a replacement of flexion by extension. These lapses are recovered from, but tend to recur more and more. Finally, an irregular phasic tremor of the muscles is all that remains. It is not the flexor muscles themselves which tire out, for these, when under fatigue of the flexion-reflex they contract no longer for that reflex, contract in response to the scratch-reflex which also employs them.
Similar results are furnished by the scratch-reflex, with certain differences in accord with the peculiar character of its individual charge. One of these latter is the feature that the individual beats of the scratch-reflex usually become slower and follow each other at slower frequency.- Also the beats, instead of remaining fairly regular in amplitude and frequency, tend to succeed in somewhat regular groups. The beats may disappear altogether for a short time, and then for a short time reappear, the stimulus continuing all the while. Here, again, the phenomena are not referable to the muscle, for when excited through other reflex channels, or through its motor nerve directly, the muscle shows its contraction well. Part of the decline of these reflexes under electrical stimulation in, the" spinal " dog may be due to reduction of the intensity of the stimulus itself by physical polarization. That does not account in the main for the above described effects. The graphic record of fatigue of the flexion of the scratch-reflex obtained by continued mechanical stimulation does not appreciably differ from that yielded under electrical stimulation. The different speed of the decline due to fatigue proceeds characteristically in different kinds of reflex, and in the same kind of reflex under different physiological conditions, e.g. " spinal shock ": this indicates its determination by other factors than electrical polarization. Polarization has in a number of cases been deferred as far as possible by using equalized alternate shocks applied in opposite directions through the same gilt needle; this precaution has not yielded: results differing appreciably from those given by ordinary double shocks or by series of make or break shocks of the same direction. The slowing of the beat in fatigue is also against the explanation by polarization, since merely weakening the stimulus does not lead to a slower beat.
When the scratch-reflex elicited from a spot of skin is fatigued,
the fatigue holds for that spot, but does not implicate the reflex as obtained from the surrounding skin. The reflex is, when tired out to stimuli at that spot, easily obtainable by stimulation two or more centimetres away. This is seen with either mechanical or electrical stimuli. When the spot stimulated second is close to the one tired out, the reflex shows some degree of fatigue, but not that degree obtaining for the original spot. This fatigue may be a local fatigue of the nerve-endings in the spot of skin stimulated, to which in experiments making use of electric stimuli some polarization may be added. Yet its local character does not at all necessarily imply its reference to the skin. It may be the expression of a spatial arrangement in the central organ by which reflex arcs arising in adjacent receptors are partially confluent in their approach toward the final common path, and are the more confluent the closer together lie their points of origin in the receptive field. The resemblance between the distribution of the incidence of this fatigue and that of the spatial summation previously described argues that the seat of the fatigue is intraspinal and central more than peripheral and cutaneous; and that it affects the afferent part of the arc inside the spinal cord, probably at the first synapse. Thus, its incidence at the synapse Ra—Pa and at R—P would explain its restrictions, as far as we know them, in the scratch-reflex.
The local fatigue of a spinal reflex seems to be recovered from with remarkable speed, to judge by observations on the reflexes of the limbs of the " spinal " dog. A few seconds' remission of the stimulus suffices for marked though incomplete restoration of the reaction. In a few instances there may be seen return of a reflex even during the stimulation under which the waning and disappearance of the reflex occurred. The exciting stimulus has usually in such cases been of rather weak intensity. In the writer's experience these spinal reflexes fade out sooner under a weak stimulus than under a strong one. This seeming paradox indicates that under even feeble intensities of stimulation the threshold of the reaction gradually rises, and that it rises above the threshold value of the weaker stimulus before it reaches that of a stronger stimulus. The scratch-reflex which has ceased to he elicited by a weak stimulus is immediately evoked—often without any sign of fatigue in its motor response—by increasing the intensity of the stimulus applied at the same electrode. The occurrence of fatigue earlier under the weaker stimulus than under the stronger also shows that the fatigue consequent under the weaker stimulus may often be, relatively to the production of the natural discharge, greater than when a stronger stimulus is employed. This, which has been of frequent occurrence in the writer's observations on the leg of the "spinal" dog, if obtaining widely in reflex actions, has evident practical importance.
It is easy to avoid in some degree the local fatigue associated with excitation of the scratch-reflex from one single spot in the skin by taking advantage of the spatial summation of stimuli applied at different points in the receptive field. When this was done, a curious result met the writer. The provocation of the reflex has been made through ten separate points in the receptive field, the distance between each member of the series of points and the point next to it being about four centimetres. Each point is stimulated by a double-induction shock delivered twice a second. When this is done a series of scratch movements is elicited, and continues longer than when the stimuli are applied at the same interval, not to succeeding series of skin points but to one point. Thus three or four hundred heats can be elicited in unbroken series. But the series tends somewhat abruptly to cease. If, then, in spite of the cessation of the response, the stimulation be continued without alteration during three or four minutes or more, the scratching movement breaks out again from time to time and gives another series of beats, perhaps longer than the first. These experiments indicate that physical polarization at the stigmatic electrode is not answerable for the fading out of the scratch-reflex. It shows also the complexity of the central mechanisms involved in the reflex. The phenomenon recalls Lombard's phases of briskness and fatigue in series of records obtained with the ergograph.
It is interesting to note certain differences between the cessation of a reflex under fatigue and under inhibition. The reflex ceasing under inhibition is seen to fade off without obvious change in the frequency of repetition of the beats, or in the duration of the individual beats. The reflex ceasing under fatigue is seen to show a slower rhythm and a sluggish course for the latter beats, especially for the terminal ones.
Among the signs of fatigue of a reflex action are several suggesting that in it the command over the final common path exercised for the time being by the receptors and afferent path in action becomes less strong, less steady and less accurately adjusted. Under prolonged excitation their hold upon the final common path becomes loosened. This view is supported by the fact that its connexion with the final common path is then more easily cut short and ruptured by other rival arcs competing with it for the final common path in question. The scratch-reflex interrupts the flexion-reflex more readily when the latter is tired out than when it is fresh.
In the hind limb of the " spinal " dog the extensor-thrust is inelicitable during the flexion-reflex. That is to say, when the flexion-reflex is evoked with fair or high intensity the writer has never succeeded in evoking the extensor-thrust, `though the flexed posture of the limb is itself a favouring circumstance for the production of the thrust if the flexion be a passive one. But when the flexion-reflex is kept up by appropriate stimulation of a single point over a prolonged time, so that it shows fatigue, the extensor-thrust becomes again elicitable. Its elicitability is, then, not regular nor facile, but it does become obtainable, usually in quite feeble degree at first, later more powerfully. In other words, it can dispossess the rival reflex from a common path when that rival is fatigued, though it cannot do so when the rival action is fresh and powerful.
Again, the crossed extension-reflex cannot inhibit the reflexion of the flexor-reflex under ordinary circumstances if the intensity of the stimulation of the competing arcs be approximately equal; but it can do so when the flexion-reflex is tired.
The waning of a reflex under long-maintained excitation is one of the many phenomena that pass in physiology under the name of fatigue. It may be that in this case the so-called fatigue is really nothing but a negative induction. Its place of incidence may lie at the synapse. It seems a process elaborated and preserved in the selective evolution of the neural machinery. One obvious use attaching to it is the prevention of the too prolonged continuous use of a common path by any one receptor. It precludes one receptor from occupying for long periods an effector organ to the exclusion of all other receptors. It prevents long continuous possession of a common path by any one reflex of considerable intensity. It favours the receptors taking turn about. It helps to ensure serial variety of reaction. The organism, to be successful in a million-sided environment, must in its reaction be many sided. Were it not for such so-called fatigue, an organism might, in regard to its receptivity, develop an eye, or an ear, or a mouth, or a hand or leg, but it would hardly develop the marvellous congeries of all those various sense-organs which it is actually found to possess.
The loosening of the hold upon the common path by so-called fatigue occurs also in paths other than those leading to muscle and effector organs. If instead of motor effects sensual are examined, analogous phenomena are observed. A visual image is more readily inhibited by a competing image in the same visual field when it has acted for some time than when it is first perceived (W. Macdougall).
One point, on a priori grounds, is a natural corollary from the " principle of the common path," as indicated by the experimental findings relative to the incidence of fatigue. The reflex arcs, each a chain of neurones, converge in their course so as to impinge upon and conjoin in links (neurones) common to whole varied groups—in other words, they conjoin to common paths. This arrangement culminates in the convergence of many separately arising arcs in the final efferent-root neurone. This neurone thus forms the instrument for many different reflex arcs and acts. It is responsive to them in various rhythm and in
various grades of intensity. In accordance with this, it seems from experimental evidence to be relatively indefatigable. It thus satisfies a demand that the principle of the common path must make regarding it.
3. In the transition from one reflex to another a final common path changes hands and passes from one master to another. A intenat(y. fresh set of afferent arcs becomes dominant on the
supersession of one reflex by the next. Of all the conditions determining which one of competing reflexes shall for the time being reign over a final common path, the intensity of reaction of the afferent arc itself relatively to that of its rivals is probably the most powerful. An afferent arc that strongly stimulates is caeteris paribus more likely to capture the common path than is one excited feebly. A stimulus can only establish its reflex and inhibit an opposed one if it have intensity. This explains why, in order to produce examples of spinal inhibition, recourse has so frequently been made in past times to strong stimuli. A strong stimulus will inhibit a reflex in progress, although a weak one will fail. Thus in Goltz's inhibition of micturition in the " spinal " dog a forcible squeeze of the tail will do it, but not, in the present writer's experience, a weak squeeze. So, likewise, any condition which raises the excitability and responsiveness of a nervous arc will give it power to inhibit other reflexes, just as it would if it were excited by a strong stimulus. This is much as in the heart of the Tunicate. There the prepotent spot whence starts the systole lies from time to time at one end and from time to time at the other. The pre-potent region at one end which usually dominates the common path is from time to time displaced by local increase of excitability at the other under local distension of the-bloodsinuses there.
In judging of intensity of stimulus the situation of the stimulus in the receptive field of the reflex has to be remembered. One and the same physical stimulus will be weak if applied near the edge of the field, though strong if applied to the focus of the field.
Crossed reflexes are usually less easy to provoke, less reliable of obtainment, and less intense than are direct reflexes. Consequently we find crossed reflexes usually more easily inhibited and replaced by direct reflexes than are these latter by those former. Thus the crossed stepping-reflex is easily replaced by the scratch-reflex, though its stimulus be continued all the time, and though the scratch-reflex itself is not a very potent reflex. But the reverse can occur with suitably adjusted intensity of stimuli.
Again, the flexion-reflex of the dog's leg is, when fully developed, accompanied by extension in the opposite leg. This crossed extensor movement, though often very vigorous, may be considered as an accessory and weaker part of the whole reflex, of which the prominent part is flexion of the homonymous limb. When the flexion-reflex is elicitable poorly, as, for instance, in spinal shock or under fatigue or weak excitation, the crossed ex-tension does not accompany the homonymous flexion and does not appear. But, where the flexion-reflex is well developed, if not merely one but both feet be stimulated simultaneously with stimuli of fairly equal intensity, steady flexion at knee, hip and ankle results in both limbs, and extension occurs in neither limb. The contralateral part of each reflex is inhibited by the homolateral flexion of each reflex. In other words, the more intense part of each reflex obtains possession of the final common paths at the expense of the less intense portion of the reflex. But if the intensity of the stimuli applied to the right and left feet be not closely enough balanced, the crossed extension of the reflex excited by the stronger stimulus is found to exclude even the homonymous flexion that the weaker stimulus should and would otherwise evoke from the leg to which it is applied.
It was pointed out above that in a number of cases the transference of control of the final common path FC from one afferent arc to another is reversible. The direction of the transference can caeteris paribus be easily governed by making the stimulation of this receptor or that receptor the more intense.
A factor largely determining whether a reflex succeed another or not is therefore intensity of stimulus.
4. A fourth main determinant for the issue of the conflict between rival reflexes seems the functional species of the reflexes. Reflexes initiated from a species of receptor appa- Spectesof ratus that may be termed noci-ceptive appear to Reflex.
particularly dominate the majority of the final common paths issuing from the spinal cord. In the simpler sensations we experience from various kinds of stimuli applied to our skin there can be distinguished those of touch, of cold, of warmth and of pain. The adequate stimuli for the first-mentioned three of these are certainly different; mechanical stimuli, applied above a certain speed, which deform beyond a certain degree the resting contour of the skin surface, seem to constitute adequate stimuli for touch. Similarly the cooling or raising of the local temperature, whether by thermal conduction, radiation, &c., are adequate for the cold and warmth sensations. The organs for these three sensations have by stigmatic stimuli been traced to separate and discrete tiny spots in the skin. In regard to skin-pain it is held by competent observers, notably by V. Frey and Kiesow, that skin-pain likewise is referable to certain specific nerve-endings. In evidence of this it is urged that mechanical stimuli applied at certain places excite sensations which from their very threshold upward possess unpleasantness, and as the intensity of the stimulus is increased, culminate in " physical pain." The sensation excited by a mechanical stimulus applied to a touch-spot does not evoke pain, however intensely applied, so long as the stimulation is confined to the touch-spot. The threshold value of mechanical stimuli for touch-spots is in general lower than it is for pain-spots; and conversely the threshold value of electrical stimuli for touch-spots is in general higher than it is for the spots yielding pain. Similarly it is said that stimulation of a cold spot or of a warm spot does not, however intense, evoke, so long as confined to them, sensations of painful quality. But pain can be excited not only by strong mechanical stimuli and by electrical stimuli, but by cold and by warmth, though the threshold value of these latter stimuli is higher for pain than for cold and warm spots. If these observations prove correct there exist, therefore, numerous specific cutaneous nerve-fibres evoking pain.
A difficulty here is that sensory nerve-endings are usually provided with sense organs which lower their threshold for stimuli of one particular kind while raising it for stimuli of all other kinds; but these pain-endings in the skin seem almost equally excited by stimuli of such different modes as mechanical, thermal conductive, thermal radiant, chemical and electrical. That is, they appear aneleclive receptors. But it is to be re-marked that these agents, regarded as excitants of skin-pain, have all a certain character in common, namely this, that they become adequate as excitants of pain when they are of such intensity as threatens damage to the skin. And we may note about these excitants that they are all able to excite nerve when applied to naked nerve directly. Now there are certain skin surfaces from which, according to most observers, pain is the only species of sensation that can be evoked. This is alleged, for instance, of the surface of the cornea—a modified piece.of skin. The histology of the cornea reveals in its epithelium nerve-endings of but one morphological kind; that is, the ending by naked nerve-fibrils that pass up among the epithelial cells. Similar nerve-endings exist also in the epidermis generally. It may therefore be that the nerve-endings subserving skin-pain are free naked, nerve-endings, and the absence of any highly evolved specialized end-organ in connexion with them may explain their fairly equal amenability to an unusually wide range of different kinds of stimuli. Instead of but one kind of stimulus being their adequate excitant, they may be regarded as adapted to a whole group of excitants, a group of excitants which has in relation to the organism one feature common to all its components, namely, a nocuous character.
With its liability to various kinds of mechanical and other damage, in a world beset with dangers amid which the individual and species have to win their way in the struggle for existence,
we may regard nocuous stimuli as part of a normal state of affairs. It does not seem improbable, therefore, that there should under selective adaptation attach to the skin a so-to-say specific sense of its own injuries. As psychical adjunct to the reactions of that apparatus we find a strong displeasurable effective quality in the sensations they evoke. This may perhaps be a means for branding upon memory, of however rudimentary kind, a feeling from past events that have been perilously critical for the existence of the individuals of the species. In other words, if we admit that damage to such an exposed sentient organ as the skin must in the evolutionary history of animal life have been sufficiently frequent in relation to its importance, then the existence of a specific set of nerves for skin-pain seems to offer no genetic difficulty, any more than does the clotting of blood or innate immunity to certain diseases. That these nerve-endings constitute a distinct species is argued by their all evoking not only the same species of sensation, but the same species of reflex movement as regards " purpose," intensity, resistance to " shock," &c. And their evolution may well have been unaccompanied by evolution of any specialized end-organ, since the naked free nerve-endings would better suit the wide and peculiar range of stimuli, reaction to which is in this case required. A low threshold was not required because the stimuli were all intense, intensity constituting their harmfulness; but response to a wide range of stimuli of different kinds was required, because harm might come in various forms. That responsive range is supplied by naked nerve itself, and would be cramped by the specialization of an end-organ. Hence these nerve-endings remained free.
It is those areas, stimulation of which, as judged by analogy, can excite pain most intensely, and it is those stimuli which, as judged by analogy, are most fitted to excite pain which, as a general rule, excite in the " spinal " animal—where pain is .of course non-existent—the pre potent reflexes. If these are reactions to specific pain-nerves, this may be expressed by saying that the nervous arcs of pain-nerves, broadly speaking, dominate the spinal centres in peculiar degree. Physical pain is thus the psychical adjunct of an imperative protective reflex. It is preferable, however, since into the merely spinal and reflex aspect of the reaction of these nerves no sensation of any kind can be shown to enter, to avoid the term " pain-nerves." Remembering that the feature common to all this group of stimuli is that they threaten or actually commit damage to the tissue to which they are applied, a convenient term for application to them is nocuous. In that case what from the point of view of sense are cutaneous pain-nerves are from the point of view of reflex-action conveniently termed noci-ceptive nerves.
In the competition between reflexes the noci-ceptive as a rule dominate with peculiar certainty and facility. This explains why such stimuli have been so much used to evoke reflexes in the spinal frog, and why, judging from them, such " fatality " belongs to spinal reflexes.
One and the same skin surface will in the hind limb of the spinal dog evoke one or other of two diametrically different reflexes according as the mechanical stimulus applied be of noxious quality or not, a harmful insult or a harmless touch. A needle-prick to the planta causes invariably the drawing up of the limb—the flexion-reflex. A harmless smooth contact, on the other hand, causes extension—the extensor-thrust above described. This flexion is therefore a noci-ceptive reflex. But the scratch-reflex—which is so readily evoked by simple light irritation of the skin of the shoulder—is relatively mildly noci-ceptive. When the scratch-reflex and the flexion-reflex are in competition for the final neurone common to them, the flexion-reflex more easily dispossesses the scratch-reflex from the final neurone than does the scratch-reflex the flexion-reflex. If both reflexes are fresh, and the stimuli used are such as, when employed separately, evoke their reflexes respectively with some intensity, in my experience it is the flexion-reflex that is usually prepotent. Yet if, while the flexion-reflex is being moderately evoked by an appropriate stimulus of weak intensity, a strong stimulus suitable for producing the scratch-reflex is applied, the steady flexion due to the flexion-reflex it replaced by the rhythmic scratching movement of the scratch.. reflex, and this occurs though the stimulus for the flexion-reflex is maintained unaltered. When the stimulus producing the scratch is discontinued the flexion-reflex reappears as before. The flexion-reflex seems more easily to dispossess the scratch-reflex from the final common paths than can the scratch-reflex dispossess the flexion-reflex. Yet the relation is reversible—by heightening the intensity of the stimulus for the scratch-reflex or lowering that of the stimulus for the flexion-reflex.
In decerebrate rigidity, where a tonic reflex is maintaining contraction in the extensor muscles of the knee, stimulation of the noci-ceptive arcs of the limb easily breaks down that reflex. The noci-ceptive reflex dominates the motor neurone previously held in activity by the postural reflex. And noci-ceptive reflexes are relatively little depressed by " spinal shock."
Noci-ceptive arcs are, however, not the only spinal arcs which in the intact animal, considered from the point of view of sensation, evoke reactions rich in affective quality. Beside those receptors attuned to react to direct noxa, the skin has others, concerned likewise with functions of vital importance to the species and colligate with sensations similarly of intense affective quality; for instance, those concerned with sexual functions. In the male frog the sexual clasp is a spinal reflex. The cord may be divided both in front and behind the brachial region without interrupting the reflex. Experiment shows that from the spinal male at the breeding season, and also at other times, this reflex is elicited by any object that stimulates the skin of the sternal and adjacent region. In the intact animal, on the contrary, other objects than the female are, when applied to that region, at once rejected, even though they be wrapped in the fresh skin of the female frog and in other ways made to resemble the female. The development of the reflex is not prevented by removal of the testes, but removal of the seminal reservoirs is said to depress it, and their distension, even by indifferent fluids, to exalt it. If the skin of the sternal region and arms is removed the reflex does not occur. Severe mutilation of the limbs and internal organs does not inhibit the reflex, neither does stimulation of the sciatic nerve central to its section. The reflex is, however, depressed or extinguished by strong chemical and pathic stimuli to the sternal skin, at least in many cases. The tortoise exhibits a similar sexual reflex of great spinal potency.
It would seem a general rule that reflexes arising in species of receptors which considered as sense-organs provoke Strongly affective sensation caeteris paribus prevail over reflexes of other species when in competition with them for the use of the "final common path." Such reflexes override and set aside with peculiar facility reflexes belonging to touch organs, muscular sense-organs, &c. As the sensations evoked 'by these arcs, e.g. " pains," exclude and dominate concurrent sensations, so do the reflexes of these arcs prevail in the competition for possession of the common paths. They seem capable of pre-eminent intensity of action.
Of all reflexes it is the tonic reflexes, e.g. of ordinary posture, that are in the writer's experience the most easily interrupted by other reflexes. Even a weak stimulation of the noci-ceptive arcs arising in the foot often suffices to lower or abolish the knee-jerk or the reflex extensor tonus of the elbow or knee. If various species of reflex are arranged, therefore, in their order of potency in regard to power to interrupt one another, the reflexes initiated in receptors which considered as sense-organs excite sensations of strong affective duality lie at the upper end of the scale, and the reflexes that are answerable for the postural tonus of skeletal muscles lie at the lower end of the scale. One great function of the tonic reflexes is to maintain habitual attitudes and postures. They form, therefore, a nervous background of active equilibrium. It is of obvious advantage that this equilibrium should be easily upset, so that the animal may respond agilely to the passing events that break upon it as intercurrent stimuli.
Results.—Intensity of stimulation, fatigue and freshness,
spinal induction, functional species of reflex, are all, therefore, physiological factors influencing the result of the interaction of reflex-arcs at a common path. It is noticeable that they all resolve themselves ultimately 'into intensity of reaction. Thus, intensity of stimulus means as a rule intensity of reaction. Those species of reflex which are habitually prepotent in inter-action with others are those which are habitually intense; those specially impotent in competition are those habitually feeble in intensity, e.g. skeletal muscular tone. The tonic reflexes of attitude are of habitually low intensity, easily interfered with and temporarily suppressed by intercurrent reflexes, these latter having higher intensity. But these latter suffer fatigue relatively early, whereas the tonic reflexes of posture can persist hour after hour with little or no signs of fatigue. Fatigue, therefore, in the long run advantageously redresses the balance of an otherwise. unequal conflict. We can recognize in it another agency working toward that plastic alternation of activities which is characteristic of animal life and increases in it with ascent of the animal scale.
The high variability of reflex reactions from experiment to experiment, and from observation to observation, is admittedly one of the difficulties that has retarded knowledge of them. Their variability, though often attributed to general conditions .of nutrition, or to local blood-supply, &c., seems far more often due to changes produced in the central nervous organ by its own functional conductive activity apart from fatigue. This functional activity itself causes from moment to moment the temporary opening: of some connexions and the closure of others. The chains of neurones, the conductive lines, have been, especially in recent years, by the methods of Golgi, Ehrlich, Apathy, Cajal and others, richly revealed to the microscope. Anatomical tracing 'of these may be likened, though more difficult to accomplish, to tracing the distribution of blood vessels after Harvey's discovery had given them meaning, but before the vasomotor mechanism was discovered. The blood vessels of an organ may be turgid at one time, constricted almost to obliteration at another. With the conductive network of the nervous system the temporal variations are even greater, for they extend to absolute withdrawal of nervous influence. Under reflex inhibition a skeletal muscle may relax to its post-mortem length, i.e. there may then be no longer evidence of even a tonic influence on it by its motor neurone. The direction of the stream of liberation of energy along the pattern of the nervous web varies from minute to minute. The final common path is handed from some group of a plus class of afferent arcs to some group of a minus class, or of a rhythmic class, and then back to one of the previous groups again, and so on. The conductive web changes its functional pattern with certain limits to and fro. It changes its pattern at the entrances to common paths. The changes in its pattern occur there in virtue of interaction between rival reflexes, " interference." As a tap to a kaleidoscope, so a new stimulus that strikes the receptive surfaces causes in the central organ a shift of functional pattern at various synapses. The central organ is a vast network whose lines of conduction follow a certain scheme of pattern, but within that pattern the details of connexion are, at the entrance to each common path, mutable. The grey matter may be compared with a telephone exchange, where, from moment to moment, 'though the end-points of the system are fixed, the connexions between starting-points and terminal points are changed to suit passing requirements, as the functional points are shifted at a great railway junction. In order to realize' the exchange at work, one must add to its purely spatial plan the temporal datum that within certain limits the connexions of the lines shift to and fro from minute to minute. An example is the " reciprocal innervation " of antagonistic muscles—when one muscle of the antagonistic couple is thrown into action the other is thrown out of action. This is only a widely spread case of the general rule that antagonistic reflexes interfere where they embouch upon the same final common paths. And that general rule is part of the general principle of the mutual interaction of reflexes that impingeupon the same common path. Unlike reflexes have successive but not simultaneous use of the common path; like reflexes mutually reinforce each other on their common path. Expressed teleologically, the common path, although economically subservient for many and various purposes, is adapted to serve but one purpose at a time. Hence it is a co-ordinating mechanism and prevents confusion by restricting the use of the organ, its minister, to but one action at a time.
In the case of simple antagonistic muscles, and in the instances of simple spinal reflexes, the shifts of conductive pattern due to interaction at the mouths of common paths are of but small extent. The co-ordination covers, for instance, one limb or a pair of limbs. But the same principle extended to the reaction of the great arcs arising in the projicient receptor organs of the head, e.g. the eye, which deal with wide tracts of musculature as a whole, operates with more multiplex shift of the conductive pattern. Releasing forces acting on the brain from moment to moment shut out from activity whole regions of the nervcus system, as they conversely call vast other regions into play. The resultant singleness of action from moment to moment is a keystone in the construction of the individual whose unity it is the specific office of the nervous system to perfect. The interference of unlike reflexes and the alliance of like reflexes in their action upon their common paths seem to lie at the very root of the great psychical process of " attention."
The spinal cord is not only the seat of reflexes whose "centres" lie wholly within the cord itself; it supplies also conducting paths for nervous reactions initiated by impulses derived from afferent spinal nerve, but involving mechanisms situate altogether headward of the cord, that is to say, in the brain. Many of these reactions affect consciousness, occasioning sensations of various kinds. In regard to the part played by spinal conduction in subserving these sensual reactions a question of practical rather than theoretical importance has been as yet the chief aim of inquiry. The inquiry has been in fact whether the impulses concerned in evoking the various species of sensations follow in their headward course along the cord certain discrete paths occupying separable fractions of the cross-area of the cord, and if they are thus confined to discrete paths in what parts of the cross-area of the cord do these parts lie. • This "localization" problem has as yet been almost the sole problem attacked, and therefore, despite its limited scope and interest,. the results attained in it may be briefly mentioned here.
Localization.—The sensations usually, grouped under the name of touch may with advantage, as shown by Head, be distinguished from the point of view of their practical elicitation into superficial and deep. The former of these are referable to stimulation of afferent nerve-fibres distributed actually to the skin, the latter to stimulation of deeper afferents subjacent to the skin. The touch-fibres belonging to the skin proper are further subdivisible, as Head has shown, into two kinds. One kind, the protopathic, yield sensations so suffused with disagree-able affective tone (skin-pain). that they may for the present purpose be considered pain-nerves, and the description of their spinal connexions be relegated to the paragraph dealing with the spinal path for pain. The other kind, the epicritic, are those which react to tangible stimuli lightly applied, such as stroking the skin with a loose pledget of cotton wool or the light touching of the skin with a pin's head or a blunt pencil point. Deep touch, on the other hand, involves afferent nerve fibres supplied by nerve-trunks not classed as cutaneous, but probably largely muscular in the sense that they run to muscles and contain side by side the afferent fibres in question and the efferent nerve-fibres causing muscular contraction. Head has brought forward clear evidence that though the afferent fibres subserving the epicritic tactual sense of the skin and deep touch of subcutaneous origin run so separate a course in the peripheral nerves, the spinal fibres constituting the intraspinal headward-running paths from these two kinds of peripheral touch-fibres, the epicritic and the deep, to the brain, lie together and are implicated together by injuries of the spinal cord. In this sense there is, therefore, in the cord a tactual path. The question
is, therefore, what course does this path follow in the cord? In the first place it must be noted that the path contains a synapse for the peripheral neurone whether belonging to the epicritic tactual group or to the deep tactual group ends in the cord, probably not far, i.e. not more than four or five segments; from its place of entrance. The rest of the headward path must therefore run through one secondary neurone at least, it may be through a series of such arranged as a headward running line of relays. It is, however, more probable that one long secondary neurone reaching the bulb covers the whole of the remaining spinal part of the trajectory. The part of the headward-running path formed by the intraspinal part of the peripheral neurone (primary afferent neurone) lies certainly in the dorsal column of the cord of the same lateral half as the side from which the neurone entered, i.e. in the right dorsal column if the neurone entered by a spinal root of the right side. The secondary neurone continuing the path lies, however,in the ventral column of the crossed half of the cord. The junction or synapse between the primary and secondary neurone lies, of course, in the grey matter of the spinal cord.
The spinal path of impulses which when they reach the brain occasion pain has been determined chiefly in regard to pain referred to the skin. The primary afferent neurones bringing these impulses to the cord are the protopathic of Head mentioned above. These, there is much evidence to show, terminate in the grey matter of the cord not far from their point of entrance into the cord, that is, they terminate intraspinally nearer their point of entrance than do the corresponding primary afferent neurones for touch. From the local spinal grey matter the pain-path is continued headward in the lateral white columns of the cord by secondary afferent neurones. These secondary afferent neurones run chiefly in the lateral column of the opposite half of the cord from that which the primary afferent neurones entered; but some run up the lateral column of the same side as that by which the primary neurones entered. The synapse between the primary afferent neurone and the secondary afferent neurone of this path lies probably in the grey matter called substantia gelatinosa of the dorsal horn.
The spinal path taken by the impulses concerned with sensations of heat and cold seems to agree closely with that taken by the impulses subserving skin pain. The position of the nerve-fibres belonging to the secondary afferent neurones of the pain and temperature path has been fairly successfully identified with that of the spinal tract called Gowers' tract. The uncrossed portion of the temperature path appears, however, to be relatively smaller as compared with its crossed portion than is that of pain.
There is much evidence that impulses contributory to " muscular sense " pass headward along the spinal cord and in their course remain for the most part uncrossed. This course would in so far agree with the course taken by the intraspinal continuations of the primary afferent neurones which form the long fibres of the dorsal columns. These are known to run to the bulb without transgressing the median plane at all. In addition to this uncrossed tract there is another, namely, that offered by the dorsal cerebellar tract, a tract of secondary neurones connected through the grey matter of the vesicular column of Clarke with primary afferent neurones of the ipselateral side. Either or both of these uncrossed tracts may be the path taken by the impulses subserving muscular sense, and there is experimental evidence in favour of such a possibility, but the question cannot be considered as definitely answered at present.
Besides the paths followed by headward-running impulses the spinal cord contains paths for impulses passing along it backwards from the brain. These paths lie almost entirely in the ventrolateral columns of the cord. The fibres of which they are composed cross but little in the cord. Their sources are various, some come from the hind brain and some from the mid brain, and in the higher mammalia, especially in man and in the anthropoid apes, a large tract of fibres in the lateral column (the crossed pyramidal tract) comes from the cortex of the neopallium of the fore brain. This last tract isthe main medium by which impulses initiated by electrical stimulation of the motor cortex reach the moto-neurones of the cord and through them influence the activity of the skeletal muscles. Of the function of the other tracts descending from the brain into the cord little is known except that mediately or immediately they excite or inhibit the spinal moto-neurones by various levels. How they harmonize one with another in their action or what their purpose in normal life may be is at present little more than conjecture. Such terms, therefore, as " paths for volition," &c., are at present too schematic in their basis to warrant their discussion here. (C. S. S.)